An experimental animal model in which the course of immunodeficiency virus infection parallels the pathogenesis of the human disease is critical for the study of human AIDS. SIV induces an immunodeficiency syndrome in infected macaques that is remarkably similar in pathogenesis to human AIDS. An important use of this animal model system is the detailed study of pathogenesis and viral determinants of disease since many studies of this type are not feasible in humans. The purpose of this project is to investigate host and viral factors involved in variable disease progression in SIV-infected macaques and the lack of disease in African primates infected with their own strains of SIV. PATHOGENESIS OF SIVsm-INFECTION OF MACAQUES.&#8232;To investigate the role of host factors in SIV-infection of macaques, we used a well-defined molecularly cloned virus (SIVsmE543-3), two macaque passages from the original sooty mangabey host. Previous studies in our lab have demonstrated that PBMC of individual macaques show vastly different susceptibility to SIVsmE543-3 infection in vitro and susceptibility is predictive of subsequent plasma viremia following inoculation. Our recent studies identified that allelic polymorphisms in the SPRY domain of TRIM5 alpha gene are responsible for much of this variation. In contrast, SIVmac appears to be adapted for resistance to rhesus macaque TRIM5. We evaluated the effect of restrictive TRIM5 alleles (TRIM-TFP and TRIM-CypA) on viremia in SIVsmE543-3-infected rhesus macaques. Restrictive genotypes were associated with significantly lower viremia than in macaques with the permissive genotype, and emergence of escape mutations in the SIV capsid protein. Two amino acid substitutions (P37S and R98S) in the capsid region were associated with escape from TRIM5TFP restriction. Introduction of these mutations into the original SIVsmE543 clone resulted in escape from TRIM5&#945; restriction in vitro and improved virus fitness in macaques with homozygous restrictive TRIMTFP alleles in vivo. Similar substitutions were observed in other SIVsm strains following transmission to macaques, collectively providing direct evidence that TRIM5&#945; exerts selective pressure on the cross-species transmission of HIV/SIV in primates. Full length infectious clones of the related challenge stock, SIVsmE660 have also been generated by close to full length RT-PCR from the virus stock. Three of these viruses replicate efficiently in rhesus PBMC in vitro and were very sensitive to neutralizing antibody (equivalent to Tier 1 classification used for HIV-1. All three are sensitive to the TRIM-TFP allele but two have mutations in the cyclophylin A binding loop that confer resistance to TRIM-CypA. Two clones were evaluated for in vivo viral replication and pathogenesis rhesus macaques following intravenous inoculation. Both clones resulted in robust and persistent viremia that was associated with significant loss of CD4+ memory T cells. Evolution of the V1 and V4 regions was associated with escape from neutralization. We have created a panel of chimeric viruses expressing later stage envelopes that range in neutralization sensitivity from Tier 1 to Tier 3 that will be evaluated in macaques as a potential challenge strains following low dose challenge. In addition, mutations associated with escape from TRIM-TFP have been introduced into one of the original clones and this will be evaluated in macaques in vivo. SIV and HIV are both associated with the development of encephalitis. For HIV, the onset of AIDS dementia is generally a late stage finding. In contrast, most models of SIV encephalitis (SIVE) use animals that progress rapidly to disease. We observed a small number of rhesus that developed SIVE over a more protracted course. Studies of the evolution of SIV revealed that terminal virus from the brain and plasma were phylogenetically distinct, consistent with compartmentalization during development of neuroAIDS. We have performed sequential intravenous passage of virus isolated from the brain of rhesus macaques with SIV encephalitis and derived a viral swarm, SIVsmH804E that induces SIV meningitis and/or encephalitis at high frequencies. We assessed viral populations in the meninges and the brain parenchyma by laser capture microdissection and observed compartmentalization of viral populations between the meninges and the parenchyma. Our results suggest that virus in the CSF may not be representative of viral populations in the brain and that caution should be applied in extrapolating between the properties of virus in these two compartments. ASYMPTOMATIC INFECTION OF NATURAL HOST SPECIES. A second goal of this project is to study the mechanisms underlying the lack of pathogenicity of SIV for their natural host species, with emphasis on SIVagm from vervet monkeys. The maintenance of a disease-free course of SIV infection in AGM likely depends on a number of mechanisms. There are a number of distinctive features of natural infection with SIV including: 1) low numbers of CD4+ T cells, 2) low expression of CCR5 at mucosal sites; 3) lack of chronic immune activation; 4) existence of a population of CD4- T cells that appear to function as helper cells but are resistant to SIV infection; and 5) low levels of maternal to infant transmission. In a collaborative study with the Brenchley lab, we found that many of the CD4+ T cells from African green monkeys down-regulate CD4 in vivo as they enter the memory pool and that this occurs independent of SIV infection. These CD4-negative memory T cells maintain functions that are normally attributed to CD4 T cells; however lack of CD4 expression apparently protects these activated cells from infection by SIVagm in vivo. Thus the absence of SIV-induced disease progression in natural hosts species may be partly explained by preservation of a subset of T cells that maintain CD4 T cell function while being resistant to SIV-infection in vivo. We have conducted small-scale breeding of SIVagm-infected AGM using pair housing; this has been successful with the production of 14 infants over a six-year period. None of these infants became infected despite being nursed by their infected mothers for a year following birth. We have initiated studies of SIVagm infection in three infant AGM (1 year of age) in collaboration with the Brenchley lab. There was no adverse clinical consequence of infection in all three infants with normal kinetics of viremia observed. Our future studies will focus on mucosal transmission of SIVagmVer and studies of the role of alternative coreceptors in addition to CCR5 for entry of SIVagm in AGM. In an initial pilot experiment with two adult AGM, we used a repetitive low dose intra rectal challenge scheme with dose escalation. Unexpectedly one AGM became infected readily at the lowest dose after two inoculations whereas the other AGM required nine inoculations and the use of 100 fold more virus. We are currently evaluating the viruses that were transmitted in these two AGM using Single genome amplification (SGA) and comparison to the inoculum. Previous studies of SIVsm infection of sooty mangabeys have shown that this virus uses an alternative coreceptor to CCR5 for infection ofin sooty mangabeys in vivo. Since AGM characteristically express very low levels of CCR5 on CD4+ T cells, it is possible that SIVagm this may be a common theme among natural hosts species. Initial studies using the CCR5 inhibitor, Maraviroc did not block infection indicating the use of alternative coreceptors in this virus model.